1. Field of the Invention
This invention relates to producing heavy or highly viscous hydrocarbons from a subsurface formation. It relates especially to a method of injecting steam in the lower portion of the formation and "heavy" oil in the upper portion. This heavy oil is also injected in a channel in the well surrounding the steam to form an insulation zone.
2. Setting of the Invention
Oil and gas are produced from underground formations through wellbores drilled from the surface to the formation. The oil and gas are contained in the pores of underground rocks or sand. The nature of the hydrocarbons found in these underground rocks or formations varies quite widely. Some of the hydrocarbons are in the form of gas. Others are in the form of oil having a low viscosity so that it will flow relatively free or easy. Still another type is the heavy or viscous oil which will hardly flow from the formation unless special means are taken to obtain such flow. Some heavy hydrocarbons are in the form of tars or bitumens. The most notable example is the solid or semisolid hydrocarbons or bitumens of the McMurray bituminous sand outcropping along the Athabasca River in Canada. The Athabasca tar is difficult to produce because it maintains a high viscosity even at high temperatures. For example, even at the 400.degree. F temperature of saturated steam at a pressure of 235 pounds per square inch, the Athabasca tar viscosity is still about 8 cps. There are many different ways which have been tried for producing the Athabasca tar. A typical representation of such methods is U.S. Pat. No. 3,155,160, F. F. Craig, Jr., et al, which teaches a method of recovery of the heavy oil by steam extraction.